1. Field of the Invention
This invention relates to a method for detecting empty component locations on a circuit board. More particularly, this invention relates to visual inspection of circuit boards for empty component locations.
2. Description of the Prior Art
Miniaturized electronic components are widely used in the electronics industry. These components are often mounted to the surface of a printed circuit board. Since miniaturized electronic components are sometimes very small, it is very difficult to determine whether the components are properly assembled on the circuit board or whether any of the components are missing.
Several methods have been used to detect missing components. One method is electronic visual inspection which is described in the Suzuki et al U.S. Pat. No. 4,549,206. In that method, the presence or absence of a component is ultimately detected by a microprocessor. A multiplicity of light beams are impinged on the circuit board and, due to the reflective properties of the component, the microprocessor discriminates between the presence and absence of a component. Electronic visual inspection such as this can be very accurate but also very expensive to obtain and maintain.
Another method for detecting the presence or absence of a component involves taking electronic measurements between nodes on a circuit board. This method has limited effectiveness when certain combinations of parallel circuits are encountered.
A third method for detecting missing components requires energization of the circuit on the circuit board. The circuit is then monitored to see if it functions properly. This method can effectively detect missing components that are critical to the circuit board's function. However, it may not detect missing components that are active when abnormal things occur in the circuit such as current surges, voltage spikes, or electromagnetic interference.
Finally, human visual inspection has been used to detect missing components. However, since some components are very small, this method can require much labor and be inefficient and unreliable, particularly as the inspector's eyes become tired.